Solve for $x$ and $y$ by deriving an expression for $y$ from the second equation, and substituting it back into the first equation. $\begin{align*}3x+9y &= -9 \\ -x-y &= 9\end{align*}$
Explanation: Begin by moving the $x$ -term in the second equation to the right side of the equation. $-y = x+9$ Divide both sides by $-1$ to isolate $y$ $y = {-x - 9}$ Substitute this expression for $y$ in the first equation. $3x+9({-x - 9}) = -9$ $3x - 9x - 81 = -9$ Simplify by combining terms, then solve for $x$ $-6x - 81 = -9$ $-6x = 72$ $x = -12$ Substitute $-12$ for $x$ back into the top equation. $3( -12)+9y = -9$ $-36+9y = -9$ $9y = 27$ $y = 3$ The solution is $\enspace x = -12, \enspace y = 3$.